Many monomers having reactive unsaturated bonds are capable of producing polymers by selecting a catalyst for cleaving the unsaturated bonds and causing a chain reaction and appropriate reaction conditions. In general, there are various monomers having unsaturated bonds, and thus various resins should be obtained. However, the number of monomers each capable of obtaining a polymer having a molecular weight of 10,000 or more and generally referred to as a polymer compound is relatively small. Typical examples of such monomers include ethylene, substituted ethylene, propylene, substituted propylene, styrene, alkylstyrene, alkoxystyrene, norbornene, various acrylates, butadiene, cyclopentadiene, dicyclopentadiene, isoprene, maleic anhydride, maleimide, fumarate ester, and an alkyl compound. A wide variety of resins are synthesized by polymerizing one kind of monomer or by copolymerizing those monomers.
Applications of those resins are mainly limited to fields of relatively inexpensive consumer appliances. The resins are hardly applied to advanced technology fields relating to electronic substrates and the like because heat resistance, thermal stability, solvent solubility, and processability cannot be attained simultaneously.
Examples of the prior documents related to the present invention include the following.
Patent Document 1: JP02-170806 A
Patent Document 2: JP 2000-128908 A
Patent Document 3: JP 2001-512752 A
Patent Document 4: U.S. Pat. No. 5,767,211
Patent Document 5: JP 2002-194025 A
Non Patent Document 1: Makromol. Chem., 1978, Vol. 179, P2069-2073
Non Patent Document 2: Makromol. Chem., 1988, Vol. 189, P723-731
Non Patent Document 3: Macromolecules, 1980, Vol. 13, P1350-1354
Non Patent Document 4: Macromolecules, 1982, Vol. 15, P1221-1225
As a method of solving such problems of a vinyl-based polymer, resin properties such as strength are improved by adding a trace amount of a polyfunctional vinyl aromatic compound such as a divinyl aromatic compound or a trivinyl aromatic compound to the vinyl-based monomer. For example, JP-A-02-170806 discloses copolymerization of a polyfunctional aromatic compound and a styrene-based monomer with heat or an initiator, to thereby obtain a styrene-based polymer having a broad molecular weight distribution and exhibiting high impact strength. However, increase in polymerization yield following the disclosed technology rapidly causes a cross-linking reaction of the polyfunctional vinyl aromatic compound. Thus, in the case where the aromatic polyfunctional vinyl compound is present in a large amount, gel formation of a resin occurs and processability and appearance of the resin are significantly degraded. Thus, conventionally performed modification of a resin with an aromatic polyfunctional vinyl compound is not sufficiently effective for applications to advanced technology fields because an addition amount of the polyfunctional vinyl aromatic compound is suppressed low to 50 to 250 ppm.
Further, JP-A-2000-128908 discloses a styrene-based polymer having a controlled branching degree by using a polyfunctional chain transfer agent in combination with a polyfunctional vinyl aromatic polymer and a method of producing the styrene-based polymer. However, an addition amount of the polyfunctional vinyl aromatic polymer is suppressed to 1 to 700 ppm with respect to an amount of a styrene-based monomer. A polymer obtained through polymerization by mixing a large amount of a polyfunctional vinyl aromatic compound usually has a highly developed cross-linking structure and often forms into an insoluble and infusible gel polymer without processability.
Meanwhile, a hyperbranched (highly branched) polymer formed of highly branched polymer chains has little entanglement of molecular chains, has a low viscosity compared with that of a linear polymer having a similar molecular weight, and has attracted attention as a highly functional polymer capable of introducing many reactive groups into branches. JP-A-2001-512752 discloses a method of producing a hyperbranched polymer through polymerization of 50 to 99.9 parts by weight of a monofunctional vinyl monomer and 0.1 to 50 parts by weight of a polyfunctional vinyl aromatic monomer at 250 to 400° C. in the presence of a radical polymerization initiator. However, results disclosed in Examples of JP-A-2001-512752 indicate that a molecular weight distribution of a polymer obtained by adding 6 to 25% of the polyfunctional vinyl aromatic compound is represented by a very high value of 60 or more because a cross-linking reaction is liable to occur during polymerization. Thus, the technique disclosed herein does not allow increase in addition amount of the polyfunctional vinyl compound and thus modification with a polyfunctional vinyl aromatic compound is not sufficiently effective for applications to advanced technology fields.
Further, U.S. Pat. No. 5,767,211 discloses a method of producing a hyperbranched polymer having no cross-linking structure through polymerization of a bifunctional to trifunctional vinyl compound in the presence of an azo-based radical polymerization initiator and a cobalt-based chain transfer catalyst. However, this polymerization method uses a chain transfer catalyst capable of accelerating β-hydrogen release for producing a branched structure, and thus a produced polymer has a structure with a double bond in a vicinity of the branched structure. Thus, even if a heat-curing operation for enhancing heat resistance of the produced polymer is performed, an effect of improving the heat resistance is small because reactivity of the polymer is low. This method has a disadvantage in that it is not appropriate for applications to advanced technology fields. In this method, a chain transfer reaction is mainly dependent on chain transfer capability of the cobalt-based chain transfer catalyst. Thus, a large amount of the chain transfer catalyst must be added to a polymerization system. This method had problems for practical use such as significantly reduced polymerization rate and difficulties in removal of the catalyst during recovery of the polymer.
Makromol. Chem. (p. 2069 to 2073, vol. 179, 1987) discloses a method of obtaining a solvent-soluble divinylbenzene polymer by performing anionic polymerization of divinylbenzene with di-iso-propylamine and butyl lithium as catalysts. Further, Makromol. Chem. (p. 723 to 731, vol. 189, 1988) discloses a method of obtaining a solvent-soluble divinylbenzene/styrene copolymer by performing anionic polymerization of divinylbenzene and styrene with lithium di-iso-propylamide as a catalyst. However, in the anionic polymerization method disclosed in those documents, selectivity of a vinyl group during polymerization is not sufficient and gel formation is liable to occur. Thus, the methods not only have problems in that a monomer concentration cannot be increased and a polymerization temperature cannot be increased to higher than 0° C., but also have problems of trace amounts of impurities in a polymerization system for progress of the polymerization. For example, water must be completely removed for progress of the polymerization. The methods have problems in industrial application in that purification of a solvent or a polymer during production involves difficulties and a polymerization reaction has low efficiency because the monomer concentration cannot be increased.
In comparison with the anionic polymerization method, a cationic polymerization method is generally known to have a small effect of impurities on polymerization and to have no problems with mixing of about 0.04 to 0.06 mole of water per 1 mole of a polymerization initiator. Thus, production of a polymer having an aromatic divinyl compound through the cationic polymerization method is expected to realize synthesis of the intended polymer without requiring advanced purification operations for the monomer and the solvent. Conventionally, Macromolecules p. 1350 to 1354, vol. 13, 1980 and Macromolecules p. 1221 to 1225, vol. 15, 1982 each disclose a method of obtaining a solvent-soluble divinylbenzene polymer by performing cationic polymerization of divinylbenzene with acetyl perchlorate as a catalyst. However, the divinylbenzene polymer to be obtained through each of the production methods disclosed in those non-patent documents has a carbon-carbon double bond in a form of an internal olefin alone. Thus, the carbon-carbon double bond has low reactivity, and a curing reaction does not proceed sufficiently in heat-curing. Thus, the divinylbenzene polymer to be obtained has a disadvantage of low heat resistance, that is, insufficient properties as a material used in advanced technology fields. The divinylbenzene polymer to be obtained through each of the production methods disclosed in those non-patent documents has a broad molecular weight distribution, and thus has disadvantages of high viscosity of a resin during molding and difficulties in forming process.
Thus, highly efficient production of a soluble polyfunctional vinyl aromatic polymer having a controlled molecular weight distribution, solving the various problems of the conventional techniques, and having a vinyl group on a pendant position by using a divinyl aromatic compound without requiring advanced purification operations for the monomer and the solvent has not been imagined.